Three distinct types of cell surface molecules play major roles in T lymphocyte activation and growth. Clonally distributed receptors provide the structural basis for selective antigen-specific regulatory and effector activity of T cells. Other non-clonally distributed molecules, present on resting T cells, appear to play a role in the triggering process. The Thy-1 molecule is one such structure. Finally, molecules appearing only on activated T cells participate in differentiation and clonal expansion, chief among these being the interleukin-2 (IL-2) receptor. To investigate the structure, function and regulated expression of these three classes of molecules, recombinant DNA clones of each have been isolated, sequenced, and used in DNA-mediated gene transfer experiments, or as hybridization probes for DNA and RNA blotting analysis of various cell types. This work has shown 1) a sequential appearance of the T cell receptor Beta, then Alpha chains during T cell ontogeny in the thymus, 2) an unusual pattern of T cell receptor gene rearrangement and expression in T suppressor cells, 3) th ability of mouse Thy-1 to act as a mitogenic signal site in human T cells, 4) the structure of the mouse IL-2 receptor, and 5) indicated a complex relationship between this structure and IL-2 receptor function.